The present invention relates to the field of the control of gear shifts in a gearbox.
More specifically, it relates to a method of synchronizing a gear on a parallel-shaft vehicle gearbox comprising at least one primary shaft connected to a power source, a secondary shaft driven by the primary shaft to transmit the motive torque to the wheels over several transmission ratios, and at least one means of coupling a gear to its shaft in order to engage a transmission ratio without mechanical synchronizing members.
This invention applies to any parallel-shaft transmission in which gear shifts with a break in transmission of torque are performed by moving a means of coupling between two transmission-ratio gears that are free to rotate on a shaft connected to the wheels of the vehicle.
It finds a nonlimiting application in a hybrid transmission for a motor vehicle provided with a combustion engine and with an electric drive machine, comprising two concentric primary shafts each one bearing at least one gear transmitting to a secondary shaft connected to the wheels of the vehicle and a first means of coupling between two primary shafts which can occupy three positions, in which positions: the combustion engine is uncoupled from the drive line connecting the electric machine to the wheels, it drives the wheels with or without top-up from the electric machine, or it is coupled to the electric machine so that their torque can be combined.
FIG. 1 describes a nonlimiting example of a hybrid transmission using this design principle. This transmission comprises a solid primary shaft 1 damping hub, connected directly by a filtration system (damper, double flywheel or the like) 2, to the inertia flywheel of a combustion engine (not depicted). The solid shaft 1 bears an idler gear 4 which can be connected therewith by a first coupling system 5 (dog clutch, synchromesh, or other type of progressive or non-progressive coupling). A hollow primary shaft 6 is connected to the rotor of an electric machine 7. The hollow shaft 6 bears two fixed gears 8, 9. It may be connected to the solid primary shaft 1 by the coupling system 5. A secondary shaft 10 bears two idler gears 11 and 12. The idler gears 11, 12 may be connected to the primary shaft by a second coupling system 13 (dog clutch, synchromesh, or other type of progressive or non-progressive coupling). The secondary shaft 10 also bears a fixed gear 14 and a gear 15 transmitting to a differential 16 connected to the wheels of the vehicle.
As mentioned earlier on, the first coupling means 5 can occupy at least three positions, in which positions:                the combustion engine is uncoupled from the drive line connecting the electric machine 7 to the wheels (sliding gear in the middle as in FIGS. 1, 2 and 3),        the combustion engine drives the wheels with or without top-up from the electric machine (sliding gear to the left), and        the combustion engine and the electric machine 7 are both coupled so that their respective torques sent to the wheels are combined (sliding gear to the right).        
In electric mode (cf. FIGS. 2 and 3), the electric machine drives the hollow primary shaft 6, while the solid shaft receives no torque from the combustion engine (position 1). The box has two electric transmission ratios referred to as “town” and “highway”, in which the torque is transmitted from the secondary shaft 10 via the fixed gears 8 or 9. To shift from one of these two ratios to the other, the box has the second coupling system 13. In the absence of synchronizing rings, a system that uses a dog clutch to couple the sliding gear with the gears requires precise control by the electric machine over the primary speed in order to avoid jerks in the torque.
Publication FR 2 933 247 discloses a method for coupling a shaft of an electric machine with a wheel shaft for an electric or hybrid vehicle. The method described involves the following steps:                the electric machine is fed a speed setpoint corresponding to the speed of the wheel shaft, disregarding the step-down ratio between the shaft of the electric machine and the wheel shaft,        when the speed of the shaft of the electric machine reaches a calibratable threshold, a zero torque is applied to it and a mechanical synchronizing device is actuated so as to equalize the speed of the shaft of the electric machine with the speed of the shaft connected to the wheels, and        as soon as the speed of the shaft of the electric machine is equal to the speed of the shaft connected to the wheels (disregarding the step-down ratio), dog-clutch engagement is performed.        
With this method, the electric machine is made to operate first of all in order to reach a speed close, but not exactly equal, to that of the shaft connected to the wheels; a synchronizing device then completes the equalizing of the speeds between the two shafts, then final dog-clutch engagement of the transmission ratio is performed.